Along with intensification of the global energy crisis, governments and vehicle companies of various countries have come to realize the main direction of the future development of automobile industry is energy-saving and emission reduction, and electric vehicle (including pure electric vehicle) development will be the best way to solve these two problems.
However, as a key technology of electric vehicles, the technology for charging battery pack still has problems existing therein now. For example, a charger for charging an electric vehicle is a typical nonlinear machine, and it will generate high harmonic currents when charging the battery pack.
A harmonic current refers to a general designation of components, frequencies of which are integer multiples of the original periodic current frequency when non-periodic current function is expanded in Fourier series. The harmonic current whose frequency is equal to an integer “n” multiple of the original periodic current frequency is referred to as an nth harmonic current. A periodic signal may be decomposed into a linear superposition of a DC component a0 and sine (or cosine) signals having different frequencies by a discrete Fourier transform (DFT):
      x    ⁡          (      t      )        =            a      0        +                  ∑                  n          =          1                ∞            ⁢                        A          n                ⁢                  cos          ⁡                      (                                          n                ⁢                                                                  ⁢                                  ω                  0                                ⁢                t                            +                              φ                n                                      )                              
wherein, An cos (nω0t+φn) is the expression of the nth harmonic, and An indicates the amplitude of the nth harmonic, whose angular frequency is nω0, and whose initial phase is φ0. When n=1, A1 cos(ω0t+ω1) is the expression of the fundamental frequency. ω0/2π is the frequency of the fundamental component, which is called the fundamental frequency, and is equal to the frequency of the AC signal.
FIG. 1(a) is the graphical representation of the fundamental wave and the harmonic after DFT, where it shows the waveform of the fundamental wave and the waveforms of the 5th and 7th harmonics.
FIG. 1(b) shows the waveforms of the harmonic distortion resulting from the superposition of the waveforms of the fundamental wave and the harmonics, on the basis of the fundamental wave and the harmonics in FIG. 1(a).
The harmonic current is very harmful to the power system, and in the meantime, the harmonic current also produces serious interferences on the electronic and communication equipments outside the power system.
If the total amount of current is set as I1, the harmonic current amount is set as IH, then THD=IH/I1×100% is called a total harmonic current distortion rate, which is an important indicator for measuring the size of the harmonic current in the power system currently.
The input current THD of the existing electric vehicle charger generally reaches 2.36% to 28%, and the THD of some on-board charger even reaches 59.6%, while a large electric vehicle charging station has many such chargers working simultaneously therein. Therefore, the THD of the harmonic currents generated in the process of charging vehicle battery by the electric vehicle charging station will increase greatly, with a very serious harm to the public grid. In all the harmonics, those which have the most significant impact on the THD are the odd harmonics, for example, 3rd, 5th, 7th harmonics and etc.